1. Field of the Invention
This invention relates to directional solidification of metal castings, most particularly to the use of seeds for epitaxial solidification.
2. Description of the Prior Art
As indicated, my copending application Ser. No. 969,130 filed Dec. 13, 1978, the use of seeds for directional solidification of articles having controlled crystallographic orientation, at least along the principal axis of solidification, is well known. To obtain an article with a crystallographic orientation derived from a seed, it is necessary to partially melt the seed and thereafter progressively solidify the article, starting from the seed, so that epitaxy results in the article. It is axiomatic that a portion of the seed must at all times be colder than the molten metal, else it would melt fully and there would be no crystal structure from which to grow. As also described in more detail in the copending application referred to above, certain production processes will result in the seed as a whole being substantially colder than both the mold which surrounds it or the molten metal which is introduced to form the article. If sufficient heat is not imparted to the seed to melt a portion of its surface, then non-epitaxial solidification will occur on its surface and a defective casting will be produced. Therefore, the prevalent practice has been to use seeds of the same composition which the article will have and as a result difficulties arise in achieving adequate melting of the seed without undesirable other effects. Therefore, there is a need for improved methods of obtaining improved seed melting.
A further problem attending the use of seeds is that surface layers of oxides or other contamination compounds form and are not readily removed by contact with molten metal. It has even been found on occasion that the portion of the seed underlying the contaminated surface layer will be melted but the surface layer itself will not have been swept away, and by its continued presence will interfere with subsequent epitaxial solidification. As a result, there is a need for preventing or readily removing contaminated surface layers on seeds in a convenient and economic manner.